This grant is concerned with the role of retroviruses in malignant transformation of host cells. Individual projects will address the characterization of integrated leukemia virus genomes and of endogenous host cell sequences (endogenous onc) related to the transforming genes of avian acute leukemia viruses. The characterization of viral RNA and protein gene products as well as a biological characterization of released viruses in both murine leukemia virus-induced thymomas and avian leukemia virus-induced bursal lymphomas. Methods will include molecular cloning of viral and host cell sequences and the use of hybridomas to generate monoclonal antibodies. The latter will be used in studies of both viral and host nuclear proteins.